This invention relates to the field of solid fuel rocket motors and the controlling of thrust in solid fuel rocket motors through the use of optically-transmitted fuel preheating.
Generally rocket motors may be classified according to their employment of liquid fuels, usually comprised of separate reservoirs of oxidizing and oxidizable materials, or solid fuels, usually comprised of a solid state mixture of oxidizing and oxidizable materials in a single fuel grain mass. Liquid fuel rockets depend upon the reaction or burning which occurs with physical contact of the oxidizing and oxidizable materials; such burning is usually initiated by a small source of external heat. Solid fuel rockets on the other hand depend upon burning which is initiated by elevating the surface temperature of the oxidizing-oxidizable mixture in at least a small region thereof to the temperature of ignition. Once ignited, the burning rate of a solid fuel rocket can be increased by preheating or raising the temperature of small areas in the fuel mass just prior to their participation in the burning reaction.
Solid fuel rockets are preferred for military and long mission uses because of their inherent simplicity and their avoidance of the complex plumbing, mixing, and control elements required in liquid fuel rockets and also because of the ease and safety with which the rocket fuel, or grain, can be handled and stored for future use. The design parameters, fuel selection tradeoffs, and operating characteristics of solid fuel rockets are well known in the art and are, for example, discussed in the textbooks "Rocket Propulsion Elements", 2d ed., by G. P. Sutton, and "Propellant Chemistry" by Stanley F. Sarver. The disclosure of these texts is hereby incorporated by reference into the present specification.
Modulation or termination of the thrust-producing reaction, once grain burn has been commenced, poses some difficulty in a solid fuel rocket; this difficulty is a characteristic of solid fuel rockets and makes it desirable to employ apparatus such as is described herein to achieve some degree of burn rate modulation or even thrust termination ability. A need for such thrust modulation or thrust termination and re-initiation can be readily appreciated in military or scientific rockets. The functions of threat avoidance, multiple purpose missions and vehicle atmospheric reentry each present a desirable environment for some form of thrust change, for example.
The present apparatus is principally concerned with thrust modulation or burn rate control during the grain burn of a solid fuel rocket. Such thrust modulation might, for example, also be desirable in tailoring the orbit of a spacecraft, in trading thrust magnitude for thrust duration in a particular rocket application, or in balancing the thrust applied to a multiple rocket vehicle--especially during the vehicle's initial liftoff, low air velocity, flight portion. Burn rate control can also be useful in achieving fixed levels of thrust which are independent of the fuel temperature variations resulting from atmospheric conditions.
The patent art includes several examples of arrangements achieving a degree of control over solid fuel rocket propulsion systems and includes the patent of J. Trotel, U.S. Pat. No. 3,457,726, which discloses a plurality of layered fuel arrangements that achieve incremental or intermittent control of the rocket thrust by way of periodically enabling the burn of new fuel increments. The Trotel apparatus contemplates the separation of fuel increments by inhibitor layers which are immune to the temperature of rocket operation and which must be violated for renewed fuel access--by the use of external energy such as heat or heat produced by electrical energy. The Trotel apparatus relies on these inert fuel-separating layers and their susceptibility to external energy as a thrust termination or periodic modulation mechanism and is unconcerned with the effect of external energy on the fuel grain per se.
The patent of Ju Chin Chu, U.S. Pat. No. 3,732,693 describes a gel fuel apparatus, wherein fuel of a class intermediate the usual liquid and solid fuel types is utilized by way of pressure feeding, preheating, and use of a granulated oxidizer in order that controllable rocket thrust be attained.
Another example of a solid fuel rocket control apparatus is found in the patent of J. E. Picquendar, U.S. Pat. No. 3,398,537, which also employs externally supplied electrical energy to maintain a supply of combustible fuel to the thrust generating reaction. The Picquendar invention employs a grain composition selected to give a non-self sustaining burn; this provides a rocket motor that is responsive to externally applied heat energy. The Picquendar apparatus contemplates the use of radiant heat from a fixed-location electrical resistance heat source and the use of this heat-based control mechanism principally for initiating and stopping thrust generation. The heater of the Picquendar apparatus is so disposed with respect to the grain as to be minimally and inconsistently effective in providing burn surface geometry change.
Another example of solid fuel rocket arrangements is found in the U.S. Patent of Hisao Okamoto et al, U.S. Pat. No. 4,369,710, which discloses an end burn arrangement of solid fuel wherein burn is enhanced through the use of heat conducting filament elements buried in the grain at manufacture and exposed to burn chamber temperatures during rocket operation. The Okamoto invention contemplates the forming of one or more cones in the fuel grain as a result of the preheating achieved with the heat conducting buried filaments. The Okamoto apparatus is principally concerned with the improvements achievable using a passive buried filament concept without extension of this concept into a modulation or control arrangement.
The patent of A. P. Adamson, U.S. Pat. No. 3,065,597, disclosed a solid fuel rocket which is capable of the extinguishing and re-ignition functions through the use of burn chamber pressure control. The Adamson invention is based on the concept of burn in the rocket pressure vessel being dependent upon the presence of pressures above a certain threshold for continuation. The Adamson apparatus provides an arrangement for increasing the burn chamber temperature by external means up to the threshold of burn maintenance when re-ignition of the rocket is desired.
Another solid fuel rocket control arrangement is found in U.S. Patent of J. J. De Haye, U.S. Pat. No. 3,529,425, which discloses the use of electrodes that locate a preheating electric arc at the grain burn surface. Alternately, the De Hayre Patent uses electrodes having a temperature-responsive variable electrical resistance that is automatically activated for preheating by approach of the grain burn surface. The De Haye apparatus also contemplates the achievement of coning at the grain burn face through increased grain burn rate and application of the externally-sourced electrical energy.
Another example of a restartable solid fuel rocket motor is found in the Patent of R. D. Wolcott, U.S. Pat. No. 3,248,875 which describes the use of electrically-heating igniter bands to reinitiate fuel grain burn.
Other examples of solid fuel rocket motor control are found in the patents of R. L. Rod, U.S. Pat. No. 3,066,482 and G. H. Messerly, U.S. Pat. No. 3,182,451, which concern respectively the achievement of increased burn rate by the addition of acoustic or other transponder-supplied elastic wave (vibratory) energy for increasing fuel combustion efficiency and the use of fluids in conductive tubes which pass through the body of the fuel grain for controlling the temperature of the fuel grain.
Additional examples of solid fuel rocket motor burn rate control are to be found in the Patents of R. H. Whitesides, Jr., U.S. Pat. No. 4,345,427, R. L. Glick, U.S. Pat. No. 3,381,476 and L. H. Caveny, U.S. Pat. No. 3,630,028. These patents concern improvement in the control apparatus employed with a rectractable filament burn rate control, variations of retractable filament structure, and the addition of grain cutter elements to the ends of retractable filaments, respectively.